Last month at a Lancaster nightclub, seven people were arrested for possession of a drug, even though the policeman leading the arrest team made it clear that the drug was not illegal. This was not the first time the police had exhibited such behaviour in relation to this drug, so what is leading to this apparently irrational police behaviour? The drug in question is mephedrone [not to be confused with the opioid substitute treatment methadone], a synthetic stimulant drug that is relatively new on the UK drug scene although it has been popular in Israel for a number of years.

Mephedrone is one of a number of so-called "legal highs" – these are drugs that users find pleasurable but which are not yet illegal, and indeed may never be. Mephedrone goes under various trade names such as "meow meow", "plant food" and "bubbles", terms derived from its chemical structure, commercial uses and subjective effects respectively. It is readily available from "head shops" and is popular with university students and other groups of clubbers. Its pharmacology is hardly studied but it is chemically related to the amphetamines. Users describe effects that suggest its actions are between those of amphetamine (speed) and MDMA (ecstasy); it activates, energises and makes them feel good but is relatively short-lasting.

There are several reasons for its current popularity. Mephedrone is sold as the pure substance, so users know what they are getting. This contrasts with current street supplies of ecstasy and speed, which are often very low quality after being cut with inactive agents and may even contain some other, more dangerous, drugs such as methylamphetamine. Another reason for its popularity is that it is legal, so can be purchased without having to make contact with drug dealers who may pressure buyers towards other drugs, and currently there is no risk of a criminal record from being caught with it. In contrast, being caught in possession of MDMA and other class A drugs means one risks up to seven years in prison, and for amphetamines [class B], five years. Users see benefits in avoiding the limitations to their careers that a prosecution for drug possession would bring. Prior to the rise of mephedrone, another stimulant known as BZP was popular, but the government has recently made this a class C drug, which may have displaced users to mephedrone.

Is mephedrone harmful? Because its use is so recent there is relatively little evidence on this point, but from its pharmacology we could not make the assumption that it would be completely safe, especially at high doses. Users report effects such as a faster heart rate as one would expect from a stimulant. In the UK, there have been scare stories of mephedrone deaths, but so far none has been proven, though mephedrone was involved in the death of a Swedish teenager in 2008. The Israel experience was that it could lead to repetitive use and stereotyped behaviours in some users consistent with the likely release of the neurotransmitter dopamine in the brain.

The European Monitoring Centre on Drugs and Drug Abuse (EMCDDA) is currently gathering Europe-wide evidence of use and harms to decide if mephedrone should be made illegal by the EU. The government would be advised to wait until this report is published, rather than rushing now to make changes to the classification system.

Last year, the Advisory Council on the Misuse of Drugs (ACMD) suggested that new drugs of uncertain harm might be put into a holding class – such as the "class D" approach adopted by New Zealand several years ago to deal with BZP with some success. Drugs in class D are allowed to be sold in limited quantities to adults, with appropriate warnings of health risks and advice on safe use. Manufacturers are licensed, provided they comply with quality control of manufacture and report sales on a regular basis. This allows an accurate knowledge of the use of the drug against which harms such as hospital presentations can be compared so that a good estimate of harm/use ratio can be obtained; an informed decision can then be made whether to make the drug illegal or not. Mephedrone would seem an ideal candidate for instigating such a holding class in the UK.